#include<bits/stdc++.h>
#define ll long long
#define ull unsigned long long
using namespace std;

const ll N = 1e6 + 10, INF = 0x3f3f3f3f;



void solve()
{
    ll n;cin >> n;
    vector<ll> a(n + 1);
    for (ll i = 1;i <= n;i++) cin >> a[i];

    ll ans = 0;
    map<ll, ll> hs;
    for (ll j = 1;j <= n;j++) {
        ans += hs[j - a[j]];
        hs[j + a[j]]++;
    }
    cout << ans;
}

signed main()
{
    //ios::sync_with_stdio(0);
    //cin.tie(0);cout.tie(0);
#define ONLINE_JUDGE
#ifndef ONLINE_JUDGE
    std::istringstream in(R"()");
    std::cin.rdbuf(in.rdbuf());
#endif
    ll T = 1;
    //cin>>T;
    for (ll i = 1;i <= T;i++) {
        solve();
    }
}